1. Field of the Invention
The present invention relates to a two-way DC-DC converter, and more particularly, to a two-way DC-DC converter which can reduce switching losses and is able to simplify the control system.
2. Description of the Related Art
For example, some of vehicles have two power supply systems with different voltages. When two-way exchange of electric power in such two direct-current (DC) power supply systems with different voltages is executed, it is common to adopt a configuration, in which a DC step-up circuit and a DC step-down circuit are disposed in parallel with each other between the DC power supply systems and the circuits are used, as necessary.
Moreover, there has been made a proposal in which, when two-way exchange of electric power between the DC power supply systems is executed, a two-way DC-DC converter is used in order to obtain an enough high DC voltage by a small-scale circuit.
For example, the Japanese Patent Application Laid-open No. 2002-165448 has disclosed a two-way DC-DC converter in which two-way type DC-AC transformation sections are provided on both sides of a transformer. Especially, a secondary DC-AC transformation section has a configuration in which a chalk coil operating as a smoothing coil is used as a chalk coil for a chopper-circuit type inverter using a choke coil at forward power transmission (step-down power transmission from a first DC terminal to a second DC terminal), and a switching and rectifying section between the chalk coil and a secondary coil of the transformer functions as a rectifier at forward-power transmission and is used as a chopper circuit at reverse-power transmission (step-up power transmission from the second DC terminal to the first DC terminal).
However, there has been a problem that the configuration in which a DC step-up circuit and a DC step-down circuit are disposed in parallel with each other between two DC power supply systems has a large scale circuit, and, at simultaneous operation, enough performance cannot be obtained due to, for example, voltage losses in the circuit.
Moreover, in the two-way DC-DC converter as disclosed in the above publication, since a switching element in the DC-AC transformation section executes ON-OFF control of a large current, and the current passes through an unsaturated range of the switching element when the large current becomes a zero current by OFF control of the switching element, an operation like an analog operation is actually is executed to cause large switching losses.
Furthermore, while driving in full synchronization between the primary DC-AC transformation section and the secondary DC-AC transformation section, there is a possibility that a large current might flow on the switching element when a current by one of the DC-AC transformation section flows and ON control of the other DC-AC transformation section is executed.